The proportion of Americans who lack health insurance fell markedly last year, according to new federal figures that provide the strongest evidence to date of how the Affordable Care Act is driving changes in health coverage across the country.

The figures, released Wednesday from a large annual Census Bureau survey that measures Americans’ well-being in several ways, found that nearly 9 million more people had coverage in 2014 than the year before. The share of people uninsured throughout the year fell to 10.4 percent, from 13.3 percent in 2013.

Most of the more than 200 million Americans with private health plans still get coverage through their jobs. But of all categories of health insurance, the sharpest increase last year was among people who bought a plan on their own — including the kind of insurance policies sold through federal and state marketplaces created under the health law.

The second-largest increase was in Medicaid, the public insurance program for the poor. The law envisioned that Medicaid would expand nationwide to people with higher incomes, but a 2012 Supreme Court ruling gave each state freedom to decide whether to broaden its program. States that did so had the sharpest rise last year in their portion of insured residents, the figures show.

Edward F. Howard, executive director of the Alliance for Health Reform, said the survey is a “foggy glass” through which to gauge the Affordable Care Act’s effect. Still, he said, the growth in people covered through individual health plans is “very heartening.” And, without the law’s impetus, it is unlikely that Medicaid would be growing at a time when the economy is improving, he added.

“The signs are pointing in the right direction,” Howard said.

Yet the figures provide a darker picture of Americans’ financial well-being. They show that the typical American’s earnings remained stagnant, and the country made no gains in narrowing the huge income gulf between the rich and the poor.

For U.S. households, median wages last year were $53,657 — essentially the same as in 2013, after accounting for inflation, and 6.5 percent less than in 2007, before the Great Recession.

Meanwhile, the nation’s concentration of wealth showed no change. The top 5 percent of Americans accounted for more than one-fifth of all income in 2014, while the bottom 20 percent earned just 3 percent of total income. The gap between high earners and low earners was essentially unchanged from the previous year and remained at its broadest point since 1993, the earliest year for which comparable data exists.

This fresh evidence of stubborn income inequality comes as the issue of Americans’ financial well-being factors prominently in the 2016 presidential campaign. Candidates are arrayed along a wide spectrum, from Sen. Bernie Sanders (I-Vt.), a progressive who has branded the wealth gap as immoral, to New York City businessman Donald Trump, a Republican billionaire who is proposing to raise taxes on the rich.

The health insurance changes have political significance, too. Five years after the Affordable Care Act’s passage, congressional Republicans still are trying to discredit or dismantle it. And the Obama administration is trying to persuade the 20 states that have balked at expanding Medicaid — led by GOP governors or legislatures — to change their minds.

Increasing health coverage to more Americans was the law’s main purpose. From the outset, though, it was clear that it would take years to assess whether it was succeeding at that goal.

Other surveys lately have indicated that more Americans have gained insurance. But another question that the new report helps to answer is how much of this expanded coverage flows from the law and how much is because of the nation’s recovery from the recession — with more people getting jobs that come with health benefits. Employer-based coverage did not change significantly last year.

With respect to Medicaid, the results point to the checkerboard pattern that has emerged as a result of the Supreme Court ruling. States where economies grew in 2013 and 2014 also saw an outsize drop in their uninsured rate, but a Washington Post analysis shows Medicaid expansion played a major role for many, even after taking economic and employment factors into account.

On average, the uninsured rate dropped 1.1 percentage points more in states that expanded Medicaid than in those that did not. The statistical analysis found that expansion continued to be a strong factor in reducing uninsured rates even after considering states’ economic growth, employment, poverty, education and racial composition.

The latest figures make clear that the odds of having health insurance vary substantially across the country, depending on an individual’s income. About 1 in 6 people with incomes of $25,000 or less had no coverage, compared with about 1 in 20 people earning $100,000 or more.

Racial and ethnic disparities also persist. Nearly 1 in 5 Hispanics remained uninsured last year, compared with fewer than 1 in 10 whites. Such disparities lingered, even though gains in coverage were about twice as large last year for Hispanics, African Americans and Asians than for whites.